The unifying emphasis of this proposal is to elucidate the molecular pathways that regulate myocardial cell differentiation and heart tube formation during development. The specific focus is to analyze one family of transcriptional regulators, the zinc- finger transcription factors GATA4, 5, and 6, to determine their contribution to the establishment of the cardiac gene program and heart morphogenesis during development. Our hypothesis states that GATA4, 5, and 6 regulate cardiac myocyte differentiation and subsequent heart development. GATA4, 5, and 6 DNA binding factors are known regulators of differentiation-specific genes such as the alpha-myosin heavy chain, cardiac troponin-C, cardiac troponin-I, aerial natriuretic factor, and brain natriuretic peptide. Targeted disruption of the GATA4 gene results in aberrant heart tube morphogenesis, yet cardiac cells are correctly differentiated. Intriguingly, the GATA6 gene is up- regulated in GATA4 null embryos, suggesting that GATA6 compensates for GATA4 in regulating differentiated gene expression. Targeted disruption of the GATA6 gene results in embryonic lethality before gastrulation, indicating that a combinatorial disruption of GATA4 and 6 will be of no value. To uncover the greater role that the three cardiac-expressed GATA factors play in the heart we propose to: 1) generate and analyze a cardiac tissue-specific disruption of the GATA6 gene in the mouse, and 2) to cross GATA6 cardiac-targeted mice with GATA4 and GATA5 targeted mice to evaluate the overall contribution of the three GATA factors to myocardial cell differentiation and heart tube morphogenesis.